Melt the chocolate: In a heatproof bowl, melt the chopped chocolate and butter together over a double boiler (or in short bursts in the microwave). Stir until smooth. Let it cool for 5 minutes.
Whisk egg yolks and sugar: In a small bowl, whisk the egg yolks with the granulated sugar until pale and creamy, about 2 minutes.
Heat cream: In a small saucepan, heat ½ cup of the cream until steaming but not boiling. Slowly whisk it into the egg mixture to temper it, then pour everything back into the saucepan. Cook over low heat, stirring constantly, until slightly thickened (about 2–3 minutes). Do not boil.
Combine with chocolate: Pour the warm egg mixture into the melted chocolate and stir in the vanilla extract and salt. Let the mixture cool to room temperature.
Whip cream: In a clean bowl, whip the remaining 1 cup of heavy cream until soft peaks form. Gently fold the whipped cream into the cooled chocolate mixture in three additions until smooth and fluffy.
Chill: Spoon or pipe the mousse into small glasses or jars. Refrigerate for at least 2 hours, or until set.
Make the topping: Just before serving, whip ½ cup heavy cream with 1 tbsp powdered sugar until soft peaks form. Spoon or pipe onto the mousse cups.
Garnish: Top with chocolate curls, shavings, or a light dusting of cocoa powder.
